IAU Focus Meetings (GA)

FM 18: Scale-free processes in the universe

Start date/time

August 12, 2015

End date/time

August 14, 2015


United States


Edith Falgarone

Coordinating Division

Division J Galaxies and Cosmology

Other Divisions:

D, H

Co-Chairs of SOC:
Edith Falgarone (ENS and Paris Observatory)
Bruce Elmegreen (IBM Watson)


  • Universality (or not) of power-law probability distribution functions: breaks and deviations
  • Interplay between cosmic rays, magnetic fields, turbulence, and gravity
  • Interplay of dark matter (DM) and baryons: baryonic fragmentation versus DM clustering in hierarchical systems
  • Transitions from cosmological to baryonic physics
  • Insights from the physics of critical phenomena and self-organized criticality
  • Steps toward better boundary conditions and sub-grid physics in computer models


Many of the most important mass distributions in the Universe, from dark matter (DM) haloes to star clusters, stars and gas clouds, all have power laws forms suggesting universal scaling laws in which gravity is dominant, encompassing both dark and baryonic matter.

DM is scale-free on cosmological scales, and possibly on all scales. A break away from this scale-free behavior would provide a definite physical scale that could help define the properties of the unknown particles whose mass dominates the universe. Observations of such a break are complicated by baryonic processes and stellar feedback, however, which also produce definite scales. The reality of physical scales in the DM distribution and the relative importance of baryonic physics are topics of extreme importance, driven entirely by astronomical observations at the present time.

Inflationary scenarios predict that seed fluctuations for the large scale structure of the Universe follow nearly scale-free power law spectra. As several of the many proposed inflationary scenarios are now subject to strong tests from the results of precision cosmology missions (WMAP, Planck, SDSS, ...), an inventory of the mechanisms leading to power-law spectra and deviations is needed, as well as those that generate and break scale-free primordial spectra.

In the standard cosmological scenario, we live in a dark-matter, dark-energy dominated Universe. From the initial density fluctuations, gravity drives the formation of a highly structured Universe in which hierarchically collapsing clumps ('haloes') of DM, contracting filaments, and sheets, are separated by expanding voids. The collapsed DM haloes correspond to the galaxies, groups, and clusters of galaxies of the visible Universe, and their hierarchical growth is central to our understanding of structure formation. Because the DM collapse in the standard scenario is scale-free, one expects objects to form with similar internal structures. Thus the measurements of DM profiles at all observable scales, from within dwarf galaxies to the most massive galaxy clusters, and at various redshifts, are powerful tests of the cosmological scenario and the nature of DM.

Galaxy formation proceeds through gravitational instability in an expanding universe. Dark matter collapse provides the skeleton where galaxies form and assemble through hierarchical merging. The main processes governing the growth of galaxies in the universe are therefore scale-free phenomena, such as gas accretion and galaxy mergers. But many other physical processes also play a fundamental role in shaping galaxies and affecting their distribution in space, such as radiative cooling, star formation, stellar feedback, and supermassive black hole feedback. More recently, new physical processes have been considered to build a consistent theory for galaxy formation: radiation hydrodynamics, cosmic rays and magnetic fields. These processes introduce new characteristic scales in the gravo-turbulent cascade leading to the formation of galaxies.

The formation of sheets, filaments, clumps, and disks is observed both at galaxy-cluster scales and within molecular clouds, down to protostellar scales, in spite of the conventional notion that pressure gradients are not negligible in molecular clouds. This similarity is intriguing, and demanding of deeper investigation.

The energy distribution of cosmic-rays also follows one of the most impressive power laws in astrophysics. Magnetic field intensity has a power-law scaling with gas density all the way from superclusters to galactic clouds. The dynamics of interstellar matter and electron density fluctuations follow power-laws too, bearing the signatures of turbulence over many orders of magnitude in size. Last, solar and stellar flares show beautiful power-law distributions for occurrence frequency as a function of their energy.

Power laws like these imply scale-free processes, but the nature of these processes is not understood. As the observed scale-free behavior is generated in these structures without any fine tuning, these are, by definition, self-organized critical. However, models of self-organized criticality usually involve short-range interactions in open systems that are slowly driven out-of-equilibrium, while gravity is long-range, and can drive systems rapidly. This implies there may be several fundamental processes at work. Similarly, turbulence involves dynamical interactions between structures on similar scales and it yet forms a wide range of structures, and the stellar initial mass function involves interactions between near-neighbor stars in the competitive accretion model, and that ends up with a broad power law too.

Equally important are the scales where these power-laws break, such as the knee in the cosmic ray energy spectrum or the transition from two-dimensional to three-dimensional galactic turbulence. The observed deviations from exact power-laws are interesting too, as are the physical couplings between various scale-free processes, such as high-energy cosmic-ray acceleration in merging cluster galaxies.

We propose a conference on all of these scale free processes to bring together observers and theoreticians from many different fields with the goal of sharing critical insights and ideas that promote rapid progress in this field. We also propose including numerical astronomers who use scaling laws to approximate unresolved physical processes on the scale of computer resolution. It will be organized on the basis of the following themes for which we propose interesting topics and a diverse selection of speakers.


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